The development some years ago of a durable, nontoxic cement for connecting plastic and metal appliances to bone has made it possible to implant endoprosthetic devices in nearly every joint in the body, and such devices are becoming more and more widely used for reconstruction of joints that have been made painful, or in which function has been impaired, by disease or injury. A variety of designs of knee joint prostheses have been proposed, and some of those designs have proven to be highly successful after fairly considerable clinical experience.
The selection of a particular prosthesis for a knee joint is based largely on the condition of the patient's knee. In those instances in which there is little disease or injury of the bones, ligaments and other tissues, relatively simple components that require little resectioning of the bone and destruction of ligaments are preferred. At the other end of the scale are severely damaged joints which require implantation of components that will replace the condylar and patellar surfaces of the femur and most or all of the tibial plateau. Moreover, prostheses for use in severely damaged joints will often be constructed to impart stability to the joint by mechanical action of elements of the components.
The knee joint prosthesis described and shown in U.S. Pat. No. 3,837,009 (which is owned by the assignee of the present invention) is exemplary of the latter type of prosthesis. It includes a post that extends up from the tibial component into a slot in the femoral component and a pin or axle that is affixed to the femoral component and passes through a hole of carefully designed shape and size in the post. The coaction between the axle and the hole provides considerable stability by restraining movements (translational, angulational and rotational) of the tibial component relative to the femoral component. The forces transmitted between the axle and the hole can be very large, and they act at a relatively great distance from the tibial plateau; as a result, there is a fairly high degree of risk of the tibial component becoming dislodged from the tibia. A number of other known knee joint prostheses of the type that are designed to impart stability to the knee joint by mechanical action are similarly subject to failure. In some cases, the implantation of the prosthesis requires removal of a considerable amount of bone, and failure may include fracture of the remaining, weakened bone.
Between the two extremes of simple, small components (see, for example, U.S. Pat. No. 3,774,244, assigned to the assignee of the present invention) and prostheses constructed to have inherent mechanical stability is a relatively wide range of damaged knee joints in which most of the bone structure at the joint must be replaced but ligaments and other tissues that provide stability in the anatomical knee joint are undamaged or can be repaired so that essentially normal restraint and control of joint function are provided by remaining soft anatomical elements. Prostheses constructed for replacement of substantially all articulating surfaces of the bones at the joint are often referred to as "total" condylar joint prostheses, and a number of total knee joint prostheses of the condylar type have been proposed (see, for example, U.S. Pat. Nos. 3,748,662, 8,816,855 and 3,869,729). The assignee of the present invention has also developed a "total" condylar knee joint prosthesis that has been marketed for some time and is in widespread use.
Generally, a total knee joint prosthesis of the condylar replacement type includes a tibial component having a platform portion which replaces substantially all of the tibial plateau and substitutes for the anatomical tibial condylar surfaces. The femoral component has laterally spaced-apart condylar portions joined by an intercondylar bridge and a patellar surface and thus affords replacement of substantially all of the surfaces of the femur that engage the tibia and patella. Whether or not the cruciate ligaments are retained in a condylar replacement prosthesis depends on the design.
The total knee joint prostheses that have been proposed and used heretofore are generally highly successful in restoring reasonably normal function to a damaged or diseased knee joint, provided that they are of correct size and are properly implanted by the surgeon, particularly in respect of the register and reasonably precise axial location to ensure relatively normal function of the ligaments and muscles in imparting stability and function to the knee joint. On the other hand, the loss of the cruciate ligaments, which is necessary with some prostheses designs and may be required with some patients in any case, presents a higher risk of dislocation than exists in an anatomical knee joint in good condition. In addition, improper sizing or placement or the inability or failure to retain or restore adequate soft tissue support can result in impairment of function and loss of stability. For example, if after implantation, the ligaments are loose, which can occur if the joint has, in effect, shortened due to incorrect sizing or placement of the prosthesis, the prosthetic joint will be relatively unstable and subject to dislocation.